Improving mileage of automobiles is one of the crucial issues for saving energy and also quite important for mankind from the viewpoint of reducing CO2 as a countermeasure against global warming.
As a measure for saving the fuel consumption in the field of engine oil (lubricating oil for internal combustion engines), it has been known that reducing the friction loss caused by engine oil through lowering the viscosity of engine oil is effective. However, lowering the viscosity of engine oil causes problems that anti-wear property required for engine oil is lowered, and that the oil consumption increases mainly due to evaporation loss. For this reason, the present situation is that lowering viscosity is rather difficult to be put into practice.
As a countermeasure against the lowering in anti-wear property associated with lowering the viscosity of base oil, there may be considered a method of blending an additive improving load-bearing ability such as an oiliness improver and an extreme-pressure additive. There have been many proposals such as blending an organomolybdenum compound as a so-called friction-modifying additive (For example, see Patent Documents 1 and 2).
On the other hand, as a countermeasure against the increase in oil consumption due to evaporation loss, there has been known a method of using synthetic oil having a low viscosity and an extremely high viscosity index. However, already developed synthetic oils are expensive, and can not always achieve sufficient performance only by using it. In the case of using mineral base oil, no effective countermeasure has been found yet. Therefore, at present, there is no widely usable fuel-saving engine oil using a low-viscosity base oil.
It is required for engine oils that the low-temperature viscosity is low enough on starting engines and the high-temperature viscosity is sufficiently high during operating engines. In other words, smaller change in viscosity is required between low temperature and high temperature. Multigrade engine oils have emerged to achieve this objective. In the SAE (Society of Automotive Engineers) viscosity classification standard J300 for multigrade engine oils, the low-temperature grading includes 0W, 5W, 10W, 15W, 20W, and 25W, and the high-temperature grading includes 20, 30, 40, 50, and 60. In particular, as a fuel-saving engine oil using a low viscosity base oil, the target of development is an engine oil having a viscosity grade of 5W or less, especially a viscosity grade of 0W. It is expected to reduce oil consumption in using an engine oil having viscosity grade of 0W-20 or less.
In a multigrade engine oil, a viscosity index improver is blended to decrease the viscosity change with temperature. When a multigrade engine oil receives heavy shearing force in engines, simultaneously, the engine oil becomes unable to function as a multigrade oil, and in many cases, the oil consumption increases as well. Therefore, a multigrade engine oil is also required to have good stability against shearing force and stability against shearing force at high temperature.
In addition to the above required properties, an excellent oxidation stability is also required for an engine oil for the view of long operating life.    Patent Document 1: Japanese Patent Application Laid-Open No. H6-313183    Patent Document 2: Japanese Patent Application Laid-Open No. H5-163497